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Proceeding the 6th Civil Engineering Conference in Asia Region: Embracing the Future through
Sustainability
ISBN 978-602-8605-08-3
THE EFFECT OF RICE HUSK ASH ON CONCRETE DURABILITY
UNDER ACID RAIN ATTACK
I. A. Ahmad1, H. Parung2, M. W. Tjaronge3, and R. Djamaluddin4
1
Doctoral Student, Civil Engineering Department, Hasanuddin University, Makassar 90245, Indonesia,
[email protected]
2
Professor, Civil Engineering Department, Hasanuddin University, Makassar 90245, Indonesia,
[email protected]
3
Professor, Civil Engineering Department, Hasanuddin University, Makassar 90245, Indonesia,
[email protected]
4
Associate Professor, Civil Engineering Department, Hasanuddin University, Makassar 90245, Indonesia,
[email protected]
ABSTRACT
The increasing requirement of concrete material use has impacts on our environment. This is the biggest
challenge of infrastructure, because the use of ordinary Portland cement generates enormous CO2
emissions. To overcome this, the use of cement is reduced by cement replacement with other materials
that are environmental friendly, such as silica fume or slag or fly ash. Rice husk ash is an alternative
supplementary cementing material that can be useful in the concrete mix. The majority of rainwater
acidity in Indonesia has pH value below the normal pH, i.e. 5.6. The concrete durability will decrease
when associated to an acidic environment. Based on the above mention reasoning, the aim of this study is
to apply the rice husk ash in the concrete mix to improve the concrete durability from acid rain. The
compressive strength was 30 MPa, with theuse of rice husk ash are 0%, 5% and 10% by weight of
cement. The acid solutions have pH values of 4 and 3, through mixing sulphate and nitric acid solutions
to simulate acid rain environment with different acidity. The data were collected by conducting an XRD
test. The results of this study indicate that rice husk ash can be potentially used as a substitute for cement
in the concrete mix to improve the concrete durability due to acid rain attacks.
INTRODUCTION
Acid rain is a form of acid deposition. Acid deposition is the process of pollutant return such as acids
(mainly sulphate and nitrate) from the air to the earth. Acid deposition can be divided into wet and dry
deposition. Acid rain is a form of wet deposition. The main sources of acid deposition are from
anthropogenic sources, namely SO2 and nitrogen oxides (NOx). Both pollutants are generated from
industrial processes and transportation. SO2 and NOx will undergo reaction with H2O in the atmosphere to
form sulphuric acid (H2SO4) and nitric acid (HNO3) which are strong acid (Nazaruddin, 2010).
The majority of rainwater acidity in Indonesia has a pH value below the normal pH, i.e. 5.6. The concrete
Durability will be impaired when associated to an acidic environment. Cement is mainly composed of
limestone, silica, alumina, and iron oxide. Environments which are containing sulphuric acid will react
with the limestone to form larger component volume. This process causes expansion and cracking in
concrete. Some previous researchers have studied the effect of acid rain on the concrete. They indicate
that acid rain with a low acid level can damage the concrete. Kanazu et al (2001) found that there was a
linear relation between total rainfall and eroded depth on the surface of mortar specimens. No effect of
acid rain with pH more than 4.0 on the eroded depth was found irrespective of the quality of mortar. The
deterioration of acid rain on the concrete specimen is caused by both H + dissolution and SO4-2 expansion
(Xie et al, 2004). The most common corrosion products which are resulting from acid rain attack are
CaSO4.2H2O crystals and SiO2.nH2O and AL2O3.nH2O gels, and no AFt is observed in the corrosion
products. The corrosion products of acid rain attack are not affected by H + and SO4-2 concentrations, but
these concentrations possess an impact of the deterioration process and the degree of damage induced by
the cementitious materials (Chen et al, 2013).
Rice husk ash (RHA) is a cement replacement material that has been proven to make less permeable
concrete and more resistant to external influences. Ramadhansyah et al (2012) observed that RHA
replacement of cement was found effective in improving the resistance of concrete to sodium chloride
attack. The RHA contained concrete showed better compressive strength performance in sodium chloride
I. A. Ahmad, H. Parung, M. W. Tjaronge, and R. Djamaluddin
solution comparing with the control concrete specimens. The chloride resistance of RHA blended cement
concrete increased with the increasing of RHA replacement level. Concrete containing 10% and 20%
RHA replacements showed excellent durability to chloride attack. Chindaprasirt & Rukzon (2008)
explained that the resistance to chloride-induced corrosion of mortar containing pozzolan as measured by
accelerated corrosion with impressed voltage (ACTIV) is significantly improved in comparison to that of
OPC mortar. The incorporation of RHA up to 30% replacement level reduces the chloride penetration,
decreases permeability, improves strength and corrosion resistance properties. From these studies, it can
be concluded that replacement level of RHA is recommended up to 25% (Saraswathy & Song, 2007).
Meanwhile Ferraro & Nanni (2012) reveales that the measurements of porosity, coefficient of water
absorption and thermal conductivity indicate that replacement of RHA in concrete reduces the open
porosity that related to the ingress of water in the concrete, but the increasing of the closed porosity is
resulting in a reduction of thermal conductivity. The best resistance to chloride ion penetration is obtained
with 15% substitution of Portland cement by RHA. The specimens that containing RHA were found to be
more resistant to HCl solution and sulphate attack than the specimens without RHA (Rodriguez de
Sensale, 2010). The presence of RHA in the concrete mixtures caused considerable reduction in the
volume of the large pores at all ages and thereby reducing the chloride ion penetration. The incorporation
of RHA improved the resistance to acid attack compared to OPC because of the silica presents in the
RHA, which is combined with the calcium hydroxide and the amount susceptible to acid attack. From the
durability studies namely chloride permeability, acid attack, alkaline attack and sulphate attack, it has
been observed that there is an increasing in resistance up to 20% replacement of cement by RHA
(Ramasamy, 2012). The study by Ganesan et al (2008) and Gastaldini et al (2010) indicate that up to 30%
of RHA could be advantageously blended with cement without adversely affecting the strength and
permeability properties of concrete. The increase in content of RHA reduces Coulomb charge values,
resulting in concrete with higher resistant to chloride penetration. Givi et al (2010) research indicate a
significant reduction in percentage of water absorption, velocity of water absorption and also coefficient
of water absorption at all ages with ultra fine RHA particles.
Based on the results above, this study examines the effect of the RHA use as cement replacement material
in increasing the resistance of concrete against the effects of acid rain. The parameters used to calculate
the level of resistance are the amount of Ca(OH)2 and gypsum content in concrete.
MATERIALS
RHA characteristics
The RHA was produced by brick manufacturing that uses rice husk as fuel. Fig. 1 provides the X-ray
power diffraction (XRD) pattern of RHA. XRD of powdered RHA was performed using Rigaku MiniFlex
II XRD. This analysis was carried out to ascertain the mineralogical phases (amorphous or crystalline) of
the RHA. It showed a hump, showing it as amorphous, as well as peaks of SiO2, representing it as
crystalline structures, which were identified as cristobalite. It indicated that the RHA was both in
amorphous and crystalline form.
Fig. 1: X-ray diffraction of RHA
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I. A. Ahmad, H. Parung, M. W. Tjaronge, and R. Djamaluddin
Materials used in concretes
In this study, natural siliceous river sand (fineness modulus of 2.6 and specific gravity of 2.42) is used as
fine aggregate crushed granite (maximum nominal size 20 mm and specific gravity of 2.3) as coarse
aggregate. The Portland composite cement (PPC) was used as the major binder material in this study, to
product 30 MPa of strength concrete. The chemical compositions of the PCC, which was carried out
using SEM-EDX, are listed in Tab. 1. Utilization RHA with PCC reduced workability of the concrete
mixture. In order to maintain its workability, a superplaticizer is used. The type of Sikamen-LN is used
and its specific gravity is 1.20.
Tab.1: Chemical composition of PCC
Oxide
compounds
CaO
SiO2
Al2O3
FeO
SO3
K2O
Na2O
MgO
Chemical
composition (%)
60.33
21.50
8.60
3.17
2.69
1.80
0.99
0.58
METHODS
Cube concrete specimens that were 150 mm x 150 mm x 150 mm were prepared for tests. Tab. 2 reveals
the mixture proportion of concrete. The cement, sand and course aggregate content are 402 kg/m3, 705
kg/m3 and 1071 kg/m3, respectively. The control mix was prepared for PCC use. RHA replacement level
of 5% and 10%, by cement weight are used in this study. The experiment used two levels of pH which are
3 and 4.
The specimens are divided into two groups. The first and the second group were respectively immersed in
solution with pH value of 3 and 4. They were immersed with a certain wet and dry cycle until 90 days,
which was immersed for 1 day and then naturally dried at room temperature for 1 day.
Tab. 2: Mix proportions of concrete (kg/m3)
Concrete
PCC
RHA
Sand
N-3
5-3
10-3
N-4
5-4
10-4
402
381.9
361.8
402
381.9
361.8
0
20.1
40.2
0
20.1
40.2
705
705
705
705
705
705
Course
Aggregate
1071
1071
1071
1071
1071
1071
Water
Superplasticizer
Curing
187
187
187
187
187
187
0
3.8
3.8
0
3.8
3.8
Acid (pH3)
Acid (pH3)
Acid (pH3)
Acid (pH4)
Acid (pH4)
Acid (pH4)
The test was carried out on concrete specimens with and without RHA after immersion in acid solution
for 90 days. The first test was conducted to cube specimens to find compressive strength. To investigate
the specimen durability a little hardened specimen is ground into powder for XRD analysis. Here, the
Ca(OH)2 and gypsum contents will be observed.
RESULTS AND DISCUSSIONS
Result of Ca(OH)2 content
The hydration products of cement usually contain alkaline substances such as Ca(OH)2 which is
stabilizing hydrated calcium silicate (C–S–H) gel, xCaO·Al2O3·yH2O (CxAHy) and preventing the
rebar-embedded cementitious materials from corrosion. However, once the cementitious materials of
encounter acidic substances, a neutralization reaction occurs between H+ in the acidic substances and
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I. A. Ahmad, H. Parung, M. W. Tjaronge, and R. Djamaluddin
Ca(OH)2 in the cementitious materials, thus decreases the alkalinity of the cementitious materials (Chen
et al, 2013).
The RHA which is containing high silica will react to Ca(OH)2 during the cement hydration thus forms
more calcium silicate hydrate gel (C-S-H). This new C-S-H contributes to the improvement in the
strength and durability properties of concrete (Givi et al, 2010).
Fig. 2 Relation between percentage of RHA and Ca(OH)2
Fig. 2 indicates the content of Ca(OH)2 in two different acid solutions. The percentage of Ca(OH)2 in
solution with pH value of 4 is slightly decreased if the percentage of RHA increases. The trend of graph
seems to be of a linear relationship. Specimen N-4 has 9% Ca(OH)2, while 5-4 and 10-4 respectively
have 6% and 3%. On the other hand, in the solution with pH value of 3, the relationship is not linear. The
content of Ca(OH)2 in N-3 is 48% then significantly reduces in 5-3 and 10-3 to 11% and 10%,
respectively.
Result of gypsum content
After 90 days of immersion, the concrete will corrode which was characterized by the formation of
gypsum and ettringite. The content of gypsum reveals in Fig. 3, which generally describes the nonlinear
relationship for both types of acid solutions.
Fig. 3 Relation between percentage of RHA and gypsum
Fig. 3 indicates that the content of gypsum slightly decreased in the specimens with a RHA 5% but
dramatically increase in those with a RHA 10%. The percentages of gypsum in solution with pH value of
4 respectively are 12%, 11% and 59% for N-4, 5-4 and 10-4. Furthermore, in the solution with pH value
of 3, there are 13%, 4% and 31%.
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I. A. Ahmad, H. Parung, M. W. Tjaronge, and R. Djamaluddin
Discussions
After 90 days of immersion, the acid solution which contains H+ and SO42- will react with the hardened
cement paste of the concrete specimen as follows (Xie et al, 2004):
Ca(OH)2 + 2H+
1/5(5CaO.6SiO.5.5H2O)(s) + 2H+(aq)
Ca(OH)2 + SO4 2-
Ca2+ + 2H2O
(1)
2.1 H2O(aq) + Ca2+(aq) + 6/5 SiO2
CaSO4 + 2OH(3)
(2)
The above equations describe the cause of reduction in Ca(OH) 2 content. The presence of RHA, which
contains high silica, has caused a decline in Ca(OH)2 content after soaking in acid. This is because
Ca(OH)2 is bound by silica into the new CSH compounds (shown in equation 4). This case provides
answers of why the specimens with RHA have smaller content of Ca(OH) 2 than those without RHA.
SiO4 4- + 1.5 Ca(OH)2 + OH- + 2H2O
Ca1.5.Si(OH)4. 2H2O
(4)
Gypsum is produced when concrete meets acid solution, detail of the reaction is shown in equation 5
whereas continuation from reaction is in equation 3.
CaSO4 + 2H2O
CaSO4. 2H2O
(5)
CaSO4. 2H2O is the formulation of gypsum. The specimens using 5% RHA experience lower gypsum
content than that without RHA. This indicates that the specimen with 5% RHA is more resistant to acid
rain attack. This situation does not occur in specimens with 10% RHA where sticking high gypsum
content, indicating severe corrosion that occurs on the concrete. To further investigation of the reason,
study is need about the content of silica, in order to obtain the answer of which one is the dominant
reaction that will occur (equation 3 or equation 4).
The content of gypsum on specimen N-4 and N-3 is relatively the same, but not for the content of
Ca(OH)2. Specimen N-3 apparently contains Ca(OH)2 greater than N-4. This case indicates the possibility
that the gypsum has been changed to ettringite (equation 6). It can be seen in equation 6 that if the
gypsum reacts with calcium aluminate hydrates then will re-generate Ca(OH)2, thus the amount of
Ca(OH)2 increases. Therefore, we need to continue investigation of the ettringite content in the
specimens.
4CaO. Al2O3.19 H2O + 3(CaSO4. 2H2O) + 16 H2O
3CaO.Al2O3.3CaSO4.31H2O + Ca(OH)2
(6)
CONCLUSION
This study examined the durability of concrete using RHA under acid rain attack by XRD analysis. This
study results in the following conclusions:
1. The replacement of 5% RHA by cement weight can reduce Ca(OH)2 and gypsum content that indicate
more durablility under acid rain attack.
2. The replacement of 10% RHA by cement weight can produce higher content of Ca(OH)2 and gypsum
that indicates corrosion.
3. The decision about the durability level of concrete with RHA needs to be continued by considering the
content of silica and ettringite in concrete.
ACKNOWLEDGMENT
This research was supported by Ministry of Education and Culture scholarship of Indonesia. The second,
the third and the fourth authors were also grateful for helpful discussions.
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